This invention relates to optical fiber communication systems and, in particular, to communication systems employing wavelength converters to enhance the bandwidth of transmission.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical transmission waveguide for carrying the optical signals, and a receiver for detecting the signals and for demodulating the information they carry. Typically the transmission waveguide comprises a plurality of segments of optical fiber interconnected by optical components such as rare-earth doped fiber amplifiers. Increasingly, the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
An important limitation on conventional fiber systems is the limited bandwidth of the optical components used in long distance transmission. While silica optical fibers have a wide bandwidth window of low loss transmission, some optical components intermediate fiber segments, such as erbium-doped fiber amplifiers, have more narrow bandwidths preventing full utilization of the transmission fiber window.
Silica optical fibers have an absorption coefficient less than 0.4 dB/km for wavelengths between 1250 and 1650 nm, making silica fibers suitable for long haul transmission over this entire spectrum. However current systems are typically limited to the wavelength range of 1530-1560 nm, where conventional erbium-doped silica fiber amplifiers (EDFAs) perform well. While prototypical EDFAs have been demonstrated over the wavelength range from 1530-1610 nm (see for example A. K. Srivastava et al., xe2x80x9c1 Tb/s Transmission of 100 WDM 10 Gb/s Channels Over 400 KM of TrueWave(trademark) Fiberxe2x80x9d, OFC ""98 Post Deadline Paper PD10 (1998)), it is doubtful that the operating range of EDFAs will be expanded over a much wider wavelength range.
It is common for lossy elements, such as dispersion compensating fiber, gain flattening filters and variable attenuators, to be included within discrete amplifiers. The loss in these elements can exceed 20 dB such that an EDFA with a 25 dB external gain will have an internal gain of 45 dB. The noise figure of these amplifiers is typically less than 6 dB. Amplifiers will need to meet such requirements in order to be practical for many applications.
One alternative amplifier is the Raman amplifier. This amplifier can provide gain at any wavelength and has been demonstrated at 1300 nm and in the 1500 nm range (see for example P. B. Hansen et al., xe2x80x9cHigh Sensitivity 1.3 xcexcm Optically Preamplified Receiver Using Raman Amplificationxe2x80x9d, Electron. Lett., Vol. 32, p.2164 (1996) and K. Rottwitt et al., xe2x80x9cA 92 nm Bandwidth Raman Amplifierxe2x80x9d, OFC ""98 Post Deadline Paper PD6 (1998)). Disadvantageously, Raman amplifiers require high pump powers. This is particularly true for high gain amplifiers.
Another alternative amplifier is a parametric amplifier (see for example E. Desurvire, Erbium Doped Fiber Amplifiers, p.451) (Wiley, 1994). These amplifiers are typically based on four wave mixing (FWM). They have the disadvantages of requiring very high pump powers and of requiring precise control of the fiber dispersion in order to achieve phase matching over long lengths of fiber.
Four wave mixing (FWM) can also be used for wavelength conversion and spectral inversion. Proposed applications of this technology include wavelength routers (S. J. B. Yoo, xe2x80x9cWavelength Conversion Technologies for WDM Network Applicationsxe2x80x9d, J. Lightwave Technology, Vol. 14, p. 955 (1996)), optical switching, and mid-span spectral inversion (S. Watanabe et al., xe2x80x9cExact Compensation for Both Chromatic Dispersion and Kerr Effect in a Transmission Fiber Using Optical Phase Conjugationxe2x80x9d, J. Lightwave Technology, Vol. 14, p. 243 (1996)). Communication systems have been demonstrated that employ FWM for spectral inversion over broad bandwidths (e.g.  greater than 70 nm) but without signal amplification (conversion efficiency of xcx9cxe2x88x9216 dB) (S. Watanabe et al., xe2x80x9cInterband Wavelength Conversion of 320 Gb/s WDM Signal Using a Polarization-Insensitive Fiber Four-Wave Mixerxe2x80x9d, ECOC ""98 (1998)). Using small effective area fibers, conversion efficiencies of up to 28 dB over 40 nm (G. A. Nowak, et. al., xe2x80x9cLow-Power High-Efficiency Wavelength Conversion Based on Modulational Instability in High-Nonlinearity Fiberxe2x80x9d, Opt. Lett., Vol. 23, p.936 (1998)) are possible, however pump powers of 28 dBm are required. Single channel gain of xcx9c0 dB has been reported at pump power of 17 dBm in standard dispersion shifted fiber, however the fiber loss resulted in net loss of the converted signal (S. Watanabe et al., xe2x80x9cHighly Efficient Conversion and Parametric Gain of Nondegenerate Forward Four-Wave Mixing in a Singlemode Fibrexe2x80x9d, Electron. Lett., Vol. 30, p. 163 (1994)). The results to date indicate that parametric amplification alone in silica fiber using pump powers less than 30 dBm will not be able to provide the gain needed for a discrete amplifier in a conventional terrestrial communications systems. Accordingly there is a need for a new kind of optical communication system for broadband transmission.
The present invention uses wavelength conversion to increase the bandwidth of optical communication systems. In an exemplary embodiment, a combination of wavelength conversion and amplification with a discrete optical amplifier (OA) to allow communications systems to operate in wavelength bands xcexxe2x80x2 outside the gain bandwidth of the OA. A transmitter launches signal channels (xcex1xe2x80x2, xcex2xe2x80x2, . . . xcexxe2x80x2N) that are outside the gain bandwidth xcex. A wavelength conversion device upstream of the amplifier maps channels xcexxe2x80x21, xcexxe2x80x22, . . . xcexxe2x80x2N to corresponding wavelengths xcex1, xcex2, . . . xcexN within xcex. The OA directly amplifies the converted signals and a second wavelength conversion device downstream of the amplifier maps the amplified signals back to the original channels xcexxe2x80x21, xcexxe2x80x22, . . . xcexxe2x80x2N. This increases the capacity of the optical communication systems by facilitating the use of both signals that lie within the OA gain bandwidth xcex and signals that can be converted to wavelengths within xcex. Associated wavelength converters, transmitters and receivers are also described.
This approach applies not only to the use of EDFAs, but also to gain-flattening elements, dispersion-compensating fibers, variable attenuators, and any intermediate components having bandwidths smaller than the transmission fiber.